Solid polyester granules or beads are used as matting agents in paints, especially flat paints, to provide increased mar and burnish resistance. When included in flat paints, they provide outstanding resistance to wet and dry abrasion. The granules are generally added to paint formulations in the form of a dispersion in a continuous aqueous phase. In some cases the polymer granules contain a pigment, such as titanium dioxide, and in other cases they are unpigmented.
In the formulation of conventional flat decorative paints, the largest of the granules are approximately the thickness in diameter of the paint film. They protrude through the surface of the film and act as stationary ball bearings. Clothing, cleaning cloths and other objects that would normally mar the film only come into contact with the granules, and hence the paint film is very burnish resistant. This is in marked contrast to conventional flat paint films, where abrasion produces a large and unsightly localised increase in gloss.
Such solid polyester granules are commonly manufactured by the copolymerisation of an unsaturated polyester and styrene in a suspension polymerisation process. For example, solid polyester granules are often produced by first emulsifying in water a solution of unsaturated polyester in styrene which contains low levels of benzoyl peroxide and azobis(isobutyronitrile). Emulsification is achieved through use of colloid stabilisers and shear stirring the solution until a desired droplet size has been obtained. When this has been achieved, a low level of N,N-diethylaniline is typically added to the reaction solution to complete the redox couple (with benzoyl peroxide) and initiate the polymerisation reaction. The azobis(isobutyronitrile) thermal initiator assists in the latter part of the exotherm to increase conversion, but has been found to be not particularly effective at this. The reaction may or may not be stirred during this stage, but stirring has traditionally been maintained to prevent any risk of settlement of the newly formed granules.
Although solid polyester granules have been manufactured by such processes for many years, in more recent times, significant occupational health and safety issues in respect of these manufacturing techniques have become apparent. In particular:    (a) N,N-diethylaniline which is toxic and odorous. N,N-diethylaniline (DEA, CAS no. 91-66-7) is a liquid which can be absorbed through the skin resulting in surface irritation as well as causing methaemoglobinaemia with resulting anoxia and central nervous system depression. More significantly, these latter effects may arise if vapours are inhaled in significant amount (LC50 rat=1920 mg/m3/4 h). These hazards are significant in the case of DEA as it is a liquid. It is classified as a harmful/dangerous substance (class 6.1) under the Australian Code for the Transport of Dangerous Goods by Road and Rail.    (b) Azobis(isobutyronitrile) (AIBN) cannot be transported at temperatures above 25° C. by law, it cakes on storage and produces a toxic by-product (tetramethylsuccinonitrile CAS No. 3333-52-6) which persists in the granules.    (c) The levels of free styrene are higher than acceptable (1500 ppm or 0.15 wt % for clear solid granules, and 3000 ppm or 0.30 wt % for pigmented solid granules) by today's standards due to poor conversion of monomer into polymer.
The aforementioned problems, and in particular the residual monomer content, generally become more of a concern when the size of the polyester granules increases.
Recently, WO 00/43425 disclosed a less odorous/toxic initiating system for the production of solid polyester granules. The initiating system disclosed comprised a combination of a hydroxyl containing aromatic amine and a diacyl peroxide. Through use of this initiating system, it was reported that the handling of relatively toxic initiating species could be avoided or at least minimised, and the odour of unreacted monomer and the toxicity of the resultant granule slurry could be reduced.
Although there are some advantages afforded by the initiating system disclosed in WO 00/43425, the residual unreacted monomer content in the granule slurry produced using this initiating system falls well short of the residual monomer levels typically achieved in the manufacture of other polymeric paint constituents such as latex polymer binders. For example, the lowest residual free styrene level in the granule slurry produced in accordance with WO 00/43425 was about 1400 ppm, whereas levels well below 50 ppm can be obtained in the manufacture of latices in which a comparable amount of styrene is polymerised.
Accordingly, there remains an opportunity to develop an initiating system that can be used in a process for manufacturing both large and small solid polyester granules that enables the granules to be produced with relatively low levels of unreacted monomer.